1. Field of the Invention
The present invention relates to an organic electroluminescence device (hereinafter, referred to as an “organic EL device” in some cases) which can be effectively applied to a surface light source for full color display, backlight, illumination light sources and the like, or to a light source array for printers, and the like.
2. Description of the Related Art
An organic EL device is composed of a light-emitting layer or a plurality of organic layers including a light-emitting layer, and a pair of electrodes sandwiching the organic layers. An organic EL device is a device for obtaining luminescence by utilizing at least either of luminescence from excitons each of which is obtained by recombining an electron injected from a cathode with a positive hole injected from an anode to produce the exciton, or luminescence from excitons of other molecules produced by energy transmission from the above-described excitons.
Heretofore, an organic EL device has been developed by using a laminate structure from integrated layers in which each layer is functionally differentiated, whereby brightness and device efficiency have been remarkably improved. For example, it is described in “Science”, vol. 267, No. 3, page 1332, 1995 that a two-layer laminated type device obtained by laminating a positive hole transport layer and a light-emitting layer also functioning as an electron transport layer; a three-layer laminated type device obtained by laminating a positive hole transport layer, a light-emitting layer, and an electron transport layer; and a four-layer laminated type device obtained by laminating a positive hole transport layer, a light-emitting layer, a positive hole-blocking layer, and an electron transport layer have been frequently used.
However, many problems still remain for putting organic EL devices to practical use. First, there is a need to attain a high emitting efficiency, and second, there is a need to attain high driving durability. Particularly, deterioration in quality when these devices are continuously driven is an important problem.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2003-123984 proposes an attempt to dispose an electron transport layer containing two or more blended electron transporting materials between a light-emitting layer and a cathode to thereby maintain electron transport ability and to increase the thickness of the device to thereby reduce trouble such as short circuits while maintaining emitting efficiency. However, these measures fail to improve the mobility of holes, and therefore, the problem of accumulation and deterioration of holes in the vicinity of the boundary between the light-emitting layer and the electron transport layer and the problem of a deterioration from the cationic state of the electron transporting material due to holes which leak from the boundary are left unsolved, arousing concern regarding the problem that driving durability is deteriorated.
Further, JP-A No. 2003-229279 discloses an attempt to reduce deterioration of an electron transporting material by mixing a hole-trapping material in an electron transport layer disposed between a light-emitting layer and a cathode to trap holes which leak from the light-emitting layer. However, although the deterioration of the electron transporting material is reduced by this measure, the mobility of electrons and the mobility of holes are deteriorated, so that the light-emitting efficiency is not improved, and therefore, the deterioration in driving durability caused by the accumulation and deterioration of holes in the vicinity of the boundary between the light-emitting layer and the electron transport layer cannot be sufficiently reduced.
It is a very important problem and a problem that always needs improvement, to attain high emitting efficiency and high driving durability at the same time in designing a practically useful luminescence device.